The thin springs can be done, but aren't really necessary if you use two pieces of thick leather on the outside. I have some clothing grade elkskin left over from a project, and I can make a couple of nice small ball bags, using the soft elk, but putting two strips of thicker cowhide from another project on the top of the "neck".
I have one of those soft ball bags, that seems like it can carry 50 or more .50 cal. balls in it at one time. Who needs to be dragging around that much lead? Certainly, for hunting, a ball block makes the most sense, but since they are not HC according to some, a small ball bag made like this that can hold 10 or few balls makes a lot of sense.
The " Secret" of keeping the balls in this bag, even upside down, is to make the neck BELOW the two wide strips LONG ENOUGH to blouse out from the weight of the balls, when the bag is held upside down. And, keep the strips of thick leather WIDE ENOUGH to hold the weight of the balls.